1. Technical Field
The present invention relates to a modulator-based lightwave transmitter and, more particularly, to an arrangement including means for controlling the operation of the modulator.
2. Prior Art
Semiconductor lasers are frequently used as optical sources for lightwave transmission systems. Modulation methods for lasers generally fall into two categories: (1) direct modulation, where the bias current applied to the laser is itself modulated by an electrical data signal; or (2) indirect (external) modulation, where the bias current to the laser is held constant, and the constant light output from the laser is modulated by the electrical data signal to provide the lightwave output data signal. Direct modulation appears to be limited to data rates less than 1 Gb/s, since inherent characteristics of the laser structure result in frequency chirping at multi-gigabit transmission rates. Thus, external modulation appears to be the preferred alternative for multi-gigabit applications.
A common arrangement for providing external modulation of an optical signal is to utilize a Mach-Zehnder interferometer. The constant optical output from the laser is coupled into the optical input waveguide of the interferometer. The electrical data signal is applied to the electrical inputs of the interferometer such that the optical output replicates the data pattern of the electrical input. Specifically, the optical input signal is split between two waveguide paths formed in a bulk optical material, such as lithium niobate. The applied electrical data signal is coupled to electrodes formed on the bulk material surface. The data signal on the electrodes varies the electric field in the vicinity of the waveguides supporting the optical signal. The presence of the electric field modifies the optical path length of each waveguide, resulting in a phase difference between the optical signals traveling along the two waveguide paths. By proper control of the phase difference, the two optical signals may exit the interferometer either completely in phase (indicating a logical "1", for example), or out of phase (indicating a logical "0", for example).
In theory, external modulation appears to provide the ideal solution to the frequency chirping problem associated with direct modulation. However, long-term stability of interferometers is questionable. The characteristics of such an interferometer, in particular, its transfer curve (i.e., optical output as a function of applied voltage) is known to drift as a function of time, temperature, and various other parameters. Any drift in transfer curve necessarily moves the bias point voltage and results in introducing error into the output optical data signal. A need thus remains in the art for a method of stabilizing the performance of an interferometer when utilized as an external modulator with a laser transmitter.